1. Field of the Invention
The invention relates to an image reading apparatus for reading an original image in an image scanner, a facsimile apparatus, or the like. More particularly, the invention is suitable for use in an image reading apparatus with a construction such that an original is irradiated by an irradiation light of an LED (light emitting diode) and an image of the original is photographed by a CCD (charge coupled device) one-dimensional image sensor.
2. Related Background Art
As a conventional color image reading apparatus, there is known a color image sensor of a light source switching type such that LEDs having light emitting characteristics of three colors of R, G, and B are used and signals are extracted from image sensors each time the same position of an original is irradiated by lights of R, G, and B, thereby obtaining a color signal corresponding to the original.
FIGS. 17 to 19 show an example of such a color image sensor of the light source switching type. The color image sensor is constructed by: a light source 53 for allowing LED lights of three colors of R, G, and B to enter from an edge surface of an optical guide member and irradiating the uniform light from a side surface to an original; an image pickup device array 29 of a short focal point; and a sensor array 1 in which a plurality of line sensors are arranged in a line.
A main portion of the color image sensor as mentioned above has a construction such that a transparent glass plate 61 which is in contact with the original surface is attached to an upper surface of a frame 60, an emission light 12 of the optical guide light source 53 provided in the frame 60 is reflected by the original surface which is in contact with an upper surface of the transparent glass plate 61, and a reflection light 13 enters the sensor array 1 provided on a board 19 through the optical system 29 in correspondence to the optical system 29. As an optical system, a short focal point image pickup device array represented by, for example, a trade name "SELFOC lens array" (made by Nippon Sheet Glass Co., Ltd.) is used.
As shown in FIG. 19, the sensor array 1 serving as photoelectric converting means is a multichip type line sensor in which a plurality of line sensors 2-1, 2-2, . . . , and 2-15 are arranged in a line on the board 19. The whole sensor array 1 is covered by a protecting film 26. Like a SELFOC lens array as mentioned above, when the reflection light from the original is formed as an image onto the sensor array 1 at an equal magnification and the image is read out, a length of sensor array 1 corresponding to only a width of original to be read is needed. Therefore, the length of sensor array 1 which is necessary changes in dependence on a size of original to be read. The number of line sensors constructing the sensor array 1 also changes.
For example, in case of the sensor unit which can read an original of the A3 size, now assuming that a length of one line sensor is equal to 20 mm, it is sufficient to construct the sensor array of 15 line sensors. The board 19 on which such a sensor array 1 is installed is supported onto a bottom plate 65 which is in engagement with the frame 60. The board 19 is connected to a flexible board 23 through a flexible wiring 28. A connector 22 for inputting/outputting a power source, a control signal, and the like is provided on the flexible board 23. The flexible board 23 is attached to the frame 60.
FIG. 20 shows a shape in the longitudinal direction of the image sensor of the optical guide light source 53 and a positional relation between the optical guide and LED boards 42 and 43. The LED boards 42 and 43 are attached to incident surfaces 54 at both edges.
FIGS. 21 and 22 show an arrangement of LED packages 71 to 74, 77 and 78 on the LED boards 42 and 43 and an arrangement of LED chips 31 to 34, 37, and 38 in LED packages 71 to 74, 77, and 78. One LED chip is enclosed in one LED package. One LED chip is included in each LED board every light emitting color of R, G, and B. There are the LED chips 31 and 32 each having a light emitting color of R, the LED chips 33 and 34 each having a light emitting color of G, and the LED chips 37 and 38 each having a light emitting color of B. On the LED boards 42 and 43, the light on/off operations of the LED chips 31 to 34, 37, and 38 can be controlled at independent timings for every light emitting color of R, G, and B.
FIG. 23 shows a cross sectional view of the optical guide light source 53 and a positional relation between the light source 53 and the LED packages 71 to 74, 77, and 78 on the LED boards 42 and 43. Rectangles shown by broken lines indicate positions of the LED packages 71 to 74, 77, and 78 on the LED boards 42 and 43 shown in FIGS. 20 and 21.
The apparatus is designed in a manner such that the lights emitted from the LED chips 31 to 34, 37, and 38 on the LED boards 42 and 43 don't directly enter a reflecting portion 56 provided in the lower portion of the optical guide light source 53 and an optical guide portion 55 is set to an angle such that the LED lights are totally reflected at both edges in the transversal direction of the optical guide portion 55. Therefore, the internal reflection is repeated many times in the optical guide light source 53 and the light is transmitted in the longitudinal direction of the optical guide portion 55 with a very small loss of light amount. After completion of a few times of internal reflection, when the light enters the reflecting portion 56, the light is diffused and reflected in the original direction and is further converged by a light converging portion 57, thereby irradiating only a region near the reading surface of the original. In this instance, the light beam entering the reflecting portion 56 is an indirect light reflected in the optical guide light source 53. An opening is adjusted in the longitudinal direction so that the irradiation light to the original becomes uniform. Therefore, a uniformity of the illuminance on the original surface is good.
When a color original is read by the image sensor with such a construction, first, data to correct a shading which is caused by a variation in sensitivity of the line sensor or a variation in irradiation light of the light source is fetched. When the data for correcting the shading is fetched, the LEDs 31 and 32 of R, the LEDs 33 and 34 of G, and the LEDs 37 and 38 of B are respectively sequentially driven so as to emit the light every kind and every color, a white reference provided in the original reading apparatus is read, and output signals of the image sensor in this instance are respectively temporarily stored into the memory. A correction is performed in a manner such that when the white reference is again read by signals for correcting the shading of R, G, and B obtained as mentioned above, the signals of R, G, and B are uniform on one line, and there is a relation of (r=g=b) among a sensor output signal r when the LEDs 31 and 32 of R emit the lights, a sensor output signal g when the LEDs 33 and 34 of G emit the lights, and a sensor output signal b when the LEDs 35 and 36 of B emit the lights.
When the original is actually read, in case of the color image sensor of the light source switching type, as mentioned above, in order to obtain the three signals of R, G, and B at one point on the original to be read, it is necessary to individually irradiate the lights of R, G, and B to the original. As a method for this purpose, the original can be read by what is called an area-sequential method such that the operation to sub-scan the image sensor for the whole original in a light-on state of the LED of one of the three colors of R, G, and B is repeated three times while changing the kind of LED to be lit on. Or, the original can be read by what is called a line-sequential method such that the image sensor is sub-scanned for the whole original while sequentially lighting on the LEDs of the three colors of R, G, and B every line to be read, thereby obtaining R, G, and B signals. By using any one of the above methods, the R, G, and B signals of the whole original surface are obtained and a color image can be reproduced by using those RGB signals.
When the optical guide light source in which the LED boards are attached to both edges is used as a light source of the color image sensor of the light source switching type, the whole original surface can be uniformly irradiated by a small number of LED chips, so that a cheap color image sensor can be realized.
However, according to such a conventional color image sensor as mentioned above, since the position of only R among the positions of the three kinds of LED packages of R, G, and B on the LED boards attached to both edges of the optical guide light source for the reflecting portions of the optical guide light source differs from those of G and B, even if an irradiation light distribution is adjusted by the shape of the aperture of the optical guide light source, it is difficult to obtain the uniform irradiation lights for all of the three colors of R, G, and B over the whole original surface. FIG. 24 shows output signals of the sensor array when the LEDs of R, G, and B are lit on and the white reference is irradiated in the light source switching type color image sensor as mentioned above. It will be understood that a uniformity in the original surface and a correlation of the irradiation lights of R, G, and B deteriorate in portions, particularly, near both edges of the optical guide light source. This is because the position of the LED light source for the reflecting portion of the optical guide light source exerts an influence on the irradiation light to the original at positions, particularly, near both edges of the optical guide light source.
Therefore, according to the conventional light source switching type color image sensor, the size in the longitudinal direction of the image sensor is set to be larger than the size of original to be actually read and the image sensor in which both edge portions of the optical guide light source are excluded has to be used for reading. Such a structure becomes a cause of obstructing a miniaturization of the color image sensor.